Circuit arrangement for the processing of physiological measurement signals

ABSTRACT

Circuit arrangement for the processing of physiological measurement signals. In a known circuit arrangement, each one of a plurality of electrodes for picking up physiological measurement signals connects with a first input connection of an input amplifier associated with the pertinent electrodes; the input amplifiers connect, at their respective second input connections, with a common reference potential connection. To check the functional capability of the input amplifiers, a calibration pulse generator connects on the output side with one of the electrodes and the reference potential connection. In addition, to check the electrodes (1, 2, 3) and their supply lines, an additional amplifier (24) connects, by its input, with the reference potential connection (7) and, at its output, with an additional electrode (25) on the patient. By this means, a component of the calibration pulse reaches the electrodes (1, 2, 3) via the body resistance of the patient and thus permits the checking of said electrodes for a defect.

The invention relates to a circuit arrangement for the processing ofphysiological measurement signals, which are taken by means ofelectrodes on a patient, having input amplifiers which are individuallyassociated with the electrodes and which are each connected at a firstinput connection with the electrodes associated with them and at asecond input connection with a common reference potential connection. Adevice is disposed downstream of the input amplifiers on the outputside, for the formation of difference signals from the output signalseach of two selectable input amplifiers. A calibration pulse generatoris connected with one of the electrodes and the reference potentialconnection and generates a calibration pulse between the two.

DESCRIPTION OF THE RELATED ART

In such a circuit arrangement known from German Patent 2,429,955, thecalibration pulse-generator generates a calibration pulse of definedheight and duration between the input connections of the individualinput amplifiers. In a device disposed downstream of the inputamplifiers on the output side, difference signals are formed from theoutput signals of two selectable input amplifiers, so that thecalibration pulse does not appear in the pertinent difference signals,on account of the difference formation. However, if one of the inputamplifiers is defective, a pulse-type output error signal results information of the difference signals. In the known circuit arrangement,it is possible to detect defects in the input amplifiers, but notdefects, which may occur at the electrodes and in their supply lines tothe input amplifiers.

SUMMARY OF THE INVENTION

It is an object of the present invention is to improve the known circuitarrangement so that a testing of the electrodes and their supply linesto the input amplifiers is also possible.

According to the invention, this object is inventively achieved with thecircuit arrangement of the initially indicated type having an additionalamplifier connected by its input to the reference potential connection,and the output connection of the additional amplifier connected to anadditional electrode on the patient. Upon the emission of a calibrationpulse by the calibration pulse generator, a pulse component defined bythe gain of the additional amplifier is fed to the patient via theadditional electrode, while the reference potential connection is actedupon by the residual pulse component. Since all electrodes disposed onthe body of the patient electrically connect with one another via thebody impedance, the pulse component fed to the patient appears at allthese electrodes. In the course of the subsequent formation ofdifference signals from the output signals of two selected inputamplifiers, the pertinent pulse component is suppressed and accordinglydoes not appear. If, however, a defect occurs in the input amplifierwhich has participated in the formation of the different signal or inthe electrode supply line between the input amplifier and the associatedelectrode, or if the pertinent electrode has faulty contact with thebody of the patient, then this defect is expressed in the form of anoutput pulse in the difference signal.

In an advantageous refinement of the circuit arrangement according tothe invention, downstream of the outputs of the input amplifiers thereis disposed a device for digital signal processing. In this device,prior to the actual signal processing of the output signals of the inputamplifiers, the calibration pulse is subtracted and is added again tothe signals after their digital processing. In this manner, thecalibration pulse is excluded from the signal processing, so that,especially when using digital filters, these do not need to be designedto account for the calibration pulse. The frequency content of thecalibration pulse differs considerably from that of the physiologicalmeasurement signals.

BRIEF DESCRIPTION OF THE DRAWINGS

The exemplary embodiments of the apparatus for a circuit arrangement forthe processing of physiological measurement signals of the presentinvention shall be set forth in greater detail below with reference tothe drawing.

FIG. 1 shows an illustrative embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an embodiment of the present invention having three inputchannels R, L and F which lead to three electrodes 1, 2 and 3 which areplaced on the body of the patient (not shown here) and specifically onthe right (R) and left (L) arm and on the left foot (F). In addition tothese three input channels R, L and F, there are in known manner alsofurther input channels for electrodes on the thoracic wall of thepatient, which however, for the sake of clarity, are not shown here.With each one of the electrodes 1, 2 and 3 there is associated in eachinstance an input amplifier 4, 5 and 6, which is designed as adifferential amplifier with a first input connection (+) and a secondinput connection (-). In this embodiment each one of the inputamplifiers 4, 5 and 6, has a first input connection (+) connected withassociated electrode 1, 2 or 3, and second input connections (-)connected with one another to form a common reference potentialconnection 7. The input amplifiers 4, 5 and 6 connect on the output sideto a channel selector unit 8, via filter stages (not shown here) whereappropriate, with a plurality of associated differential amplifiers 9,10 and 11. The differential amplifiers 9, 10 and 11 each have two inputs12 and 13, 14 and 15, or 16 and 17. In this embodiment, the outputsignals of two input amplifiers selected by the channel selector unit 8,e.g. 4 and 6, can be fed to each input pair, e.g. 14 and 15. Thus, inthe case of the example shown, the difference signal R-F is formed atthe output 18 of the differential amplifier 10.

Between the two input connections (+) and (-) of the input amplifier 4associated with the channel R or the electrode designated by 1, acalibration pulse generator is connected 19, which generates between itstwo connections 20 and 21, on demand, a voltage pulse of defined height(preferably 1 mV) and duration. To avoid electrically loading thechannel R, the calibration pulse generator 19 is designed as animpedance converter 22 with a voltage pulse emitter 23 in the feedbackbranch. At the reference potential connection 7, which is common to allinput amplifiers 4, 5 and 6, there is therefore present a potentialwhich corresponds to the potential at the electrode 1 overlaid with thevoltage pulse of the calibration pulse generator 19. Accordingly, thispotential is overlaid with the same disturbance signals which aredetected by the electrodes 1, 2 and 3. Since the input amplifiers 4, 5and 6 are differential amplifiers, these disturbance signals are notamplified, but suppressed. When a calibration pulse is generated, thereference potential for all input amplifiers 4, 5 and 6 is altered atthe connection 7 in the same manner. Since the difference between thetwo output signals of the input amplifiers 4, 5 and 6 is formed by thedifferential amplifiers 9, 10 and 11. No. alteration of the outputsignals of the differential amplifiers 9 to 11 results from thealteration of the reference potential at the connection 7, as long asthe input amplifiers 4, 5 and 6 are operating properly. On the otherhand, if one of the input amplifiers 4, 5 and 6 is defective, then anoutput defect signal appears at that one of the differential amplifiersthat received an input signal from the defective input amplifier.

If, by way of example, the voltage generated by the calibration pulsegenerator 19 between the connections 20 and 21 is designated by U, andif the potentials taken off from the electrodes 1, 2 and 3 aredesignated by R, L and F, then the potential at the reference potentialconnection 7 has the value R-U and the input voltages U, L-R+U and F-R+Uare present between the input connections (+) and (-) of the inputamplifiers 4, 5 and 6. Thus, given the input amplifiers 4, 5 and 6 arein order, between the output signals of the input amplifiers 4 and 5 isthe difference signal L-R, between the input amplifiers 5 and 6 is thedifference signal L-F and between the input amplifiers 4 and 6 is thedifference signal F-R.

The circuit arrangement shown in FIG. 1 corresponds--as described up tothis point--to the arrangement known from German Patent 2,429,955. Asthe former description indicates, defects in the input amplifiers 4, 5and 6 may be detected, but not at the electrodes 1, 2 and 3 and theirsupply lines to the input amplifiers 4, 5 and 6. In order to achievethis, an additional amplifier 24 connects, by its input, to thereference potential connection 7 and, at its output, with an additionalelectrode 25, which is disposed on the body of the patient, e.g. on hisright foot. In this manner, the potential present at the referencepotential connection 7 is amplified and fed to the body of the patientvia the electrode 25. If, in this case, the voltage pulse generated bythe calibration pulse generator is designated by U and the gain of theamplifier 24 is indicated by x, then the patient is acted upon by apulse component of the height U.x/(x-1), while the pulse componentappearing at the reference potential connection 7 exhibits the heightU/(x-1). Since the electrodes 1, 2, 3 and 25 are electricallyconductively connected with one another via the body impedance of thepatient, the pulse component U.x/(x-1) appears at all electrodes 1, 2, 3and 25. Since, furthermore, upon the occurrence of the calibration pulsethe potential conditions at the electrodes 1, 2 and 3 alter in the samedirection and to the same extent as at the reference potentialconnection 7, as already shown hereinabove, the input voltages U, L-R+Uand F-R+U appear at the input amplifiers 4, 5 and 6, so that thedifference signals between the outputs of the input amplifiers 4 and 5,5 and 6, and 4 and 6, have the unchanged values of L-R, L-F and F-Rrespectively. If, however, a defect appears in one of the inputamplifiers 4, 5 or 6 or in one of the electrode supply lines between theinput amplifiers 4, 5 and 6 and the electrodes 1, 2 and 3, or if one ofthe electrodes 1, 2 or 3 has faulty contact with the body of thepatient, then this defect is expressed in the form of an output pulse atthat one of the differential amplifiers 9, 10 and 11 that connects withthe pertinent defective channel R, L or F via the channel selector unit8.

In the illustrative embodiment shown, the formation of differencesignals from the output signals of the input amplifiers 4, 5 and 6 takesplace in an analogous manner. The manner in which the functionalcapability of the differential amplifiers 9, 10 and 11 can also bechecked is evident from the already cited German Patent 2,429,955. Byway of an alternative to the analog signal processing, the outputsignals of the input amplifiers 4, 5 and 6 can also be further processeddigitally. Since, as has already been described hereinabove, thecalibration pulse vanishes only upon the formation of the differencesignals from the output signals of the input amplifiers. In the case ofdigital signal processing the calibration pulse would have to beprocessed as well in all process steps which precede a differenceformation. Especially in the case of digital signal filtering, thiswould be a disturbing factor, since the calibration pulse exhibits adifferent frequency spectrum from the physiological signals. Moreover,in the case of a complex signal processing, it would be necessary todraw a distinction as to whether the calibration pulse vanishes orremains otherwise preserved on account of difference formations.Accordingly, after conversion of the output signals of the differentialamplifiers 9, 10 and 11 into digital form in an A/D converter 26, thedigital signals are supplied to a subtraction unit 27 wherein thecalibration pulse is subtracted from the output signals of the inputamplifiers 4, 5 and 6. After this, the digital signal processing takesplace in a digital processor 28, and at the end thereof a newcalibration pulse is added to the digitally processed signals in anaddition unit 29. In this manner, the calibration pulse is excluded fromthe digital signal processing. Only where a defect is present in theinput amplifier circuit, does a residual pulse which participates in thesignal processing remain after the subtraction.

Although various modifications may be suggested by those versed in theart, it should be understood that we wish to embody within the scope ofthe patent granted hereon, all such modifications as reasonably andproperly come within the scope of our contribution to the art.

We claim:
 1. A circuit arrangement for processing physiologicalmeasurement signals obtained by a plurality of electrodes interactingwith a patient, comprising:a plurality of electrodes a plurality ofinput amplifiers respectively individually associated with saidelectrodes, each input amplifier having a first input connected to theelectrode associated therewith and a second input connected to a commonreference potential, and each input amplifier having an output at whichthat input amplifier generates an output signal; means connected to theoutputs of said input amplifiers for forming difference signals from therespective signals at the outputs of two selected ones of said inputamplifiers; calibration pulse generator means connected to one of saidelectrodes and to said reference potential for generating a calibrationpulse between said two selected input amplifiers; an additionalelectrode an additional amplifier having an input connected to saidreference potential and an output connected to said additional electrodeadapted to interact with said patient, said calibration pulse beingsupplied to said patient via said additional amplifier and saidadditional electrode and thereby causing said calibration pulse toappear in the respective output signals of said input amplifiers as longas said electrodes and input amplifiers are operating faultlessly; andmeans connected to said outputs of said input amplifiers for digitallyprocessing said output signals, including means for subtracting saidcalibration pulse from the respective output signals of said inputamplifiers before said digital processing and for adding saidcalibration pulse to the respective output signals after said digitalprocessing.